The need to display a three-dimensional object to a viewer can occur in a variety of applications, such as in medical imaging, graphics (e.g. computer aided engineering, computer aided design and the like) or entertainment. Currently, photographic as well as presently available electronic display technologies, such as cathode ray tube (CRT) monitors, liquid crystal displays, plasma panels and the like, rely on displaying an object in only two dimensions.
Specifically, conventional photographic film is itself a flat two-dimensional medium which is unable to record three-dimensional information inherent in a scene. In particular, if one object is placed partially in front of another object in a scene and a photograph is then taken at a zero degree offset angle from the front of that scene, the resulting photograph will show a front view of that scene wherein one object is shown partially in front of the other object but will fail to reveal the separation between these two objects. The depth information inherent in the scene will simply not be recorded in this instance. By contrast, some depth information can be photographically recorded if another photograph were to be taken at a angle offset from the centerline of the image thereby providing a two-dimensional perspective image of both objects in which one dimension partially conveys the depth information. However, if the relative offset angle between these images depicted in the two photographs is excessive, then it becomes very difficult for a viewer to properly construct an adequate mental three-dimensional image of the photographed scene. Now, by comparing two photographed images of the same scene taken from slightly different perspectives, where each perspective approximately matches that for the same scene as seen by a corresponding eye of a human observer, as would be the case with stereoscopic photography, then a pair of human eyes, when simultaneously viewing the resulting photographs, is then able in conjunction with the visual center in the viewer's brain to combine the images such that the viewer will visually perceive an acceptable three-dimensional view of the scene. Unfortunately, stereoscopic cameras require two separate lenses and slaved shutter assemblies and are thus mechanically complex. Furthermore, stereoscopic cameras and associated equipment are not widely available. In addition, stereoscopic prints and slides require special viewing equipment that places both of the two photographed images, which collectively form a single stereoscopic image, at the correct separation and distance from the eyes of a viewer in order to produce a relatively accurate stereoscopic image. Unfortunately, this viewing equipment tends to be cumbersome to use and quite bulky. For these reasons, stereoscopic photography, which has been in existence for quite some time, has seen relatively little use. In addition, stereoscopic images do not provide parallax affects, i.e. where an image of an object changes in response to the changing perspective imparted to a viewer as that viewer moves his or her head from one side of the object to the other. Since parallax provides important depth cues, omission of parallax affects restricts the three-dimensional accuracy inherent in a stereoscopic image. U.S. Pat. No. 4,649,425 (issued to Pund on Mar. 10, 1987) describes an electronically based stereoscopic system in which the system monitors the position of a viewer and adjusts the current position of each individual image based upon the detected position of the viewer. This system, similar to the manual stereoscopic approaches, also fails to display parallax affects.
Electronic technologies for capturing an image of a scene predominantly rely on scanning that image using well-known two-dimensional raster scan techniques. As such, the scanned image only provides a two-dimensional view of a three-dimensional scene and thereby, in a similar fashion as a photographed image, suppresses depth information from the scanned scene. An electronic display that relies on use of two individual display elements to achieve a stereoscopic affect would suffer the same loss of parallax as with such photographic displays. Relatively new electronic imaging technologies, such as tomography and the like, are able to produce a cross-sectional view through an object, such as a cross-sectional view of a patient taken transverse to that patient's spine.
Given these limitations in the technologies currently used to capture an image and/or stereoscopically display that image, it appears that the art has endeavoured to display a three-dimensional object (or scene) primarily from a sequence of two-dimensional sectional images thereof which have either been photographically or electronically obtained rather than to display the object through stereoscopic techniques.
The simplest of all approaches known in the art that attempts to produce a three-dimensional representation of an object is simply for a viewer to arrange a "stack" of photographic images, with each image being of a different section of the object. All the sections are taken in the same direction through the object. These images would be arranged in a particular sequence with the image of an uppermost section of the object being on the top of the stack, followed by images of sections occurring at increasing depths from the top, and ending with the image of the lowest section of the object located at the bottom of the stack. With this stack, the viewer would then quickly "thumb" through these images, i.e. successively view each image in the sequence for only a fraction of a second. In this manner, the viewer, using the persistence associated with his or her vision, would, if the images are sequenced at approximately the right speed, see each image superimposed onto the previous image(s) and thereby perceive a crude three-dimensional depiction of the object. Unfortunately, this method is highly dependent upon the skill of the user. To obtain even a crude three-dimensional display, the speed at which the images are sequenced must be chosen sufficiently fast to avoid flicker but yet sufficiently slow to enable the viewer's eye to perceive and then retain each image. As one can readily appreciate, this technique is very cumbersome to use and, based upon the skill of all the viewers, produces very inaccurate and highly inconsistent results.
With this in mind, the art has turned to various techniques that attempt to mechanize the display of a sequence of photographic images in order to reliably produce three-dimensional images. One such technique, such as that disclosed in U.S. Pat. Nos. 3,428,393 (issued to de Montebello on Feb. 18, 1969) and 3,462,213 (issued to de Montebello on Aug. 19, 1969), relies on projecting each image in a pre-defined sequence of photographic "sectional" images through a rotating spirally or helically shaped projection screen that has a transmissive and diffusive surface. Unfortunately, the systems disclosed in these patents are physically large, mechanically relatively complex and, owing to the need to transmit light through a diffusive screen, exhibit a loss of image brightness. A similar system is disclosed in U.S. Pat. No. 4,294,523 (issued Oct. 13, 1981 to Woloshuk et al) in which each image in a sequence of two-dimensional "sectional" images is arranged in a film strip and then momentarily illuminated. The resulting image is then projected through a transmissive projection screen that moves in synchronism with a rate at which the individual images are illuminated. Disadvantageously, the system is mechanically large and also, by virtue of projecting an image through a screen, suffers a loss of image brightness. Another, basically similar system, though described for use as a three-dimensional radar display, is disclosed in U.S. Pat. No. 3,202,985 (issued to Perkins et al on Aug. 24, 1965 -- hereinafter referred to as the '985 Perkins et al patent) wherein a succession of individual images are projected onto a rotating spherical spiral screen where the radius of the screen at a current rotational angle and at a given moment corresponds to the angular orientation of a radar antenna at that moment. Because the system disclosed in the '985 Perkins et al patent requires a mirror, which directs a point source of illumination, to mechanically move at a relatively high speed to sweep out a volume on the rotating screen in a controlled three-dimensional manner and in synchronism with the three-dimensional movement of the radar antenna, this system is rather complex and difficult to implement. Another technique as disclosed in U.S. Pat. No. 4,297,009 (issued to Mezzrich et al on Oct. 27, 1981) relies on placing a sequence of two-dimensional images, specifically transparencies, of varying sectional views of an object along an annular region of a disk. The disk is then rotated at a relatively high speed with all images being successively illuminated one at a time and displayed on a rotating spiral screen. The resulting image seen by the viewer appears to be three-dimensional composite of the individual two-dimensional images. Unfortunately, this technique is hampered by a rather limited viewing angle.
Therefore, a specific need exists in the art for a three-dimensional display system that at least reduces, if not eliminates, the deficiencies associated with three-dimensional display systems known in the art.